Generally, equipment referred to as a power converter, inverter or drive is used to provide power to another piece of equipment such as a motor. Specifically, such a converter (converter is used generally herein to refer to converters, inverters and drives) is coupled to a utility connection to receive incoming input power such as three-phase AC power. The converter conditions the power to provide a conditioned power signal to the equipment to be powered. In this way, incoming power to the equipment may be of improved efficiency, leading to reduced costs to operate the equipment.
Multi-level power converters have been gaining popularity mainly due to improved power quality, lower switching losses, better electromagnetic compatibility, and higher voltage capability. These improvements in power conversion are achieved by using a multiple voltage step strategy. One common multi-level inverter topology is a series H-bridge inverter, in which multiple H-bridge inverters are connected in series. Since this topology consists of series power conversion cells, the voltage and power level may be easily scaled.
Typically, commercial converters are built up based on modular units, namely, power conversion cells, which are generally formed of a three-phase diode-based front-end rectifier, a DC-link capacitor bank, and a single-phase full-wave inverter. Using such cells, improved power quality at both the AC system and the motor sides can be realized.
In a three-phase inverter, the sum of three-phase instantaneous output power is almost constant if the load does not change. But in a single-phase inverter, the instantaneous output power varies with time. Hence, the output energy of a capacitor bank of the power cell also varies and causes voltage ripple in the DC bus. Therefore, a very large capacitor bank has to be used in order to secure enough energy storage in the DC-link and reduce the voltage ripple.
In-rush currents into capacitive components are a key concern in power-up stress to components. As mentioned above, large amounts of capacitance are present in medium voltage drives to reduce the voltage ripple in power cells and increase power quality at the utility side. The in-rush current to charge the capacitor banks can be extremely high. This high in-rush current can severely stress the converter's fuses, input rectifiers, transformers and power switches, and can significantly reduce the reliability and life expectancy of the modules. Industrial facilities such as manufacturing plants often have multiple supplies on a line, and the combined in-rush current can trip a circuit breaker. The resulting unplanned downtime is extremely expensive and reduces the profits.
Large in-rush currents degrade the performance and lifetime of a drive in a number of ways. The sparking of the switch contacts leads to premature switch failure; it can also cause the line circuit breaker to trip, especially if there are multiple power supplies on the same circuit; the current can thermally over-stress the input rectifiers, causing immediate power cell failure; high currents on the fuse cause heating, which can slowly degrade the fuse over time; and high in-rush current also stresses the transformer modules and reduces their life or may cause failure in windings.
Thus large in-rush current to a drive at power-up challenges designers to apply different techniques to control the capacitor in-rush current by using pre-charge circuits. The functional requirement of the pre-charge circuit is to minimize peak current flowing out from the power source by slowing down the dV/dT of the input power voltage. However, known pre-charge circuits can be complex and significantly add to component costs, increase size and weight of the drive, and can have reliability issues.